1. Field of the Invention
This invention relates to a process for the production of ceramics and more particularly, it is concerned with a process for obtaining a ceramic compact which is homogenized to give a decreased dispersion of strength with a high Weibull modulus.
2. Description of the Prior Art
Up to the present time, ceramic materials have been used as sintered or after machining the surfaces thereof in predetermined dimensions. Ceramics thus sintered, having roughened or bent surfaces, cannot be used for precision parts. Accordingly, in order to obtain a precision surface, a cutting or grinding step is carried out using a diamond tool. However, these machining methods have drawbacks in that a number of fine cracks or defects are formed on the surface of the ceramic sintered bodies or compacts, so that the properties thereof, and in particular the strength is and Weibull modulus of the ceramic bodies are lowered. The problem is more serious for materials having a lower fracture toughness.
For the purpose of solving the above described problems, it has been proposed to coat the machined surface with a ceramic material or to subject it to a heat treatment in an oxidation atmosphere, thus forming an oxidation film and eliminating the effect of surface cracks. However, in these methods, the adhesion between the substrate and the coating film or oxidation film is not sufficient and the effect of surface cracks is not eliminated.
For example, J. J. Petrovic and J. Jacobson have made studies on the relationship between the atmosphere and temperature during the heat treatment and the breaking strength by subjecting hot-press silicon nitride to grinding and then to a Knoop indentation of 2600 g, thus introducing fine cracks onto the surface of the silicon nitride, as described in "The Strength of Silicon Nitride After Exposure to Different Environments" in Ceramics for High-Performance Applications (1974), page 397-414. It is described therein that in the case of a sample with a large surface defect due to the Knoop indentation, the breaking strength is recovered by a heat treatment at a temperature of 1073.degree. K. or higher in various oxygen partial pressures or nitogen atmosphere while in the case of a sample undergoing only a grinding step, the strength is somewhat lowered by similar heat treatments. As is evident from this citation, it is very difficult to eliminate the effects of the fine machining cracks formed during grinding, apart from the large surface defects.
J. E. Ritter jr. et al. have reported efforts to eliminate the effect of cracks or fractures formed on the surface of hot-press silicon nitride as a result of grinding the surface thereof by utilizing surface oxidation in "Application of Fracture mechanics in Assuring Against Fatigue Failure of Ceramic Components" in Ceramics for High-Performance Applications--III: Reliability (1983), page 503-533, which it is described that the fine points of the cracks are blunted by oxidation, but that simultaneously, pits are formed as a result of oxidation, so that the strength is determined depending on the extent of oxidation. Since the oxidation resistance of silicon nitride varies to a great extent with the variety of oxides added as an assistant during sintering and since there are silicon nitride ceramics of such a type as being readily oxidized to markedly lower the strength thereof, the above described efforts to eliminate the effect of machining cracks by controlling the oxidation is not always suitable.